The work proposed in this Supplement Request deals with sequencing of radioactively labeled polypeptides. The latter will be synthesized in vitro by translating mRNA's in the presence of radioactively labeled amino acids. The labeled polypeptides will be purified by immunoprecipitation and by polyacrylamide gel electrophoresis in sodium dodecyl sulfate. The separated polypeptides will be electrophoretically eluted from the gel, precipitated and subjected to Edman degradation using a Beckman sequencer. Separation and detection of the radioactive PTH amino acids will be by liquid chromatography and subsequent liquid scintillation counting. The objective is to determine the amino acid sequence of amino terminal sequence extensions which we have detected in presecretory proteins and in a precursor to a chloroplast protein, synthesized in the cytoplasm. These sequence extensions are present only when proteins are synthesized in vitro by translating mRNA's. In vivo they are short-lived and are cleaved either before the synthesis of the chain is completed (presecretory proteins) or shortly thereafter (chloroplast protein). Most in vitro systems lack the proper cleavage enzyme so that the large molecules, containing the extension, could be detected readily. We have postulated that these short-lived sequence extensions function in mediating transfer of proteins across specific membranes.